Today at the Chaos Computing Congress, a group of researchers (Alex Sotirov, Marc Stevens, Jake Appelbaum, Arjen Lenstra, Benne de Weger, and David Molnar) announced that they have found a way to forge website certificates that will be accepted as valid by most browsers. This means that they can successfully impersonate any website, even for secure connections.

Let me unpack that for non-experts.

One of the cornerstones of web security is the use of secure connections. When your browser makes a secure connection to (say) Amazon and gets a page to display, the browser displays in its address bar a URL like “https://www.amazon.com”. The “https” indicates that the secure (https) protocol was used, and the browser also displays a happy blue lock or key icon to tell you the connection was secured.

The browser cooperates with Amazon’s web server to secure the connection via a two-step process. First, the two computers negotiate a shared secret key that they can use to communicate privately, using crypto tricks that I won’t describe here. Second, your browser authenticates Amazon’s web server, that is, it assures itself that the party on the other end of the connection is the genuine Amazon.com server.

Amazon has a digital certificate that it sends to your browser, as part of proving its identity. The certificate is issued by a party called a certification authority or CA. Your browser comes pre-programmed with a list of CAs its trusts; you can change the list but hardly anyone does. If your browser makes an encrypted connection to “amazon.com”, and the party on the other end of the connection owns a certificate for the name “amazon.com”, and that certificate was issued by a CA that your browser trusts, then your browser will conclude that it has a secure connection to amazon.com.

Now we can understand what the researchers accomplished: they showed how to forge a certificate corresponding to any address on the Web. For example, they can forge a certificate that allows themselves, or you, or me, or anybody, to impersonate amazon.com, or freedom-to-tinker.com, or maybe even fbi.gov. That is supposed to be impossible, for obvious reasons.

The forged certificates will say they were issued by a CA called “Equifax Secure Global eBusiness”, which is trusted by the major browsers. The forged certificates will be perfectly valid; but they will have been made by forgers, not by the Equifax CA.

To do this, the researchers exploited a cryptographic weakness in one of the digital signature methods, “MD5 with RSA”, supported by the Equifax CA. The first step in this digital signature method is to compute the hash (strictly speaking, the cryptographic hash) of the certificate contents.

The hash is a short (128-bit) code that is supposed to be a kind of unique digest of the certificate contents. To be secure, the hash method has to have several properties, one of which is that it should be infeasible to find a collision, that is, to find two values A and B which have the same hash.

It was already known how to find collisions in MD5, but the researchers improved the existing collision-finding methods, so that they can now find two values R and F that have the same hash, where R is a “real” certificate that the CA will be willing to sign, and F is a forged certificate. This is deadly, because it means that a digital signature on R will also be a valid signature on F — so the attacker can ask the CA to sign the real certificate R, then copy the resulting signature onto F — putting a valid CA signature onto a certificate that the CA would never voluntarily sign.

To demonstrate this, the researchers created a forged certificate signed by the Equifax CA. For safety, they made the forged certificate expire in the past and point to a harmless site. But it’s clear from their description that they can forge a certificate for any site they want.

Whose fault is this? Partly it’s a consequence of problems with the MD5 hash method. It’s been known for a few years that MD5 is in the process of melting down, so prudent designers have been moving away from MD5, replacing it with newer, better hash methods. Similarly, prudent CAs should not be signing certificates that use MD5-based signature methods; instead they should insist on signature methods involving stronger hashes. The Equifax CA did not follow this precaution.

The problem can be fixed, for now, by having CAs refuse to create new MD5-based signatures. But this is a sobering reminder that the certification process that underlies web site authentication — a mechanism we all rely upon daily — is far from bulletproof.